This invention relates to passive thermal control articles, and, more particularly, to paint and freestanding film articles useful in spacecraft thermal control applications.
Spacecraft are subjected to a wide range of thermal environments during service. One side of the spacecraft may face free space, while the other side faces the sun. Heat is radiated into free space to cool the spacecraft, but the spacecraft can be heated intensively in direct sunlight.
Active and passive temperature control techniques are used to maintain the interior temperature of the spacecraft, which contains persons or sensitive instruments, within acceptable operating limits. Active temperature control usually involves machinery or electrical devices, such as electrical heaters and heat pipes. The present invention deals with passive temperature control.
An established approach to passive temperature control is the use of surface coatings, typically termed "paints", on the external surface of the spacecraft, or solar films (sometimes termed "blankets") overlying the external surface of the spacecraft. A white paint or film, for example, has a low solar absorptance, while a black paint or film has a high solar absorptance. The selective application of such paints to various elements of the spacecraft exterior greatly aids in controlling its temperature. Solar films function similarly, except that they are freestanding articles that are bonded to or suspended over a surface. The present invention deals with a white paint and a white solar film that are useful in spacecraft temperature control applications.
In most cases, the paint or film article must dissipate electrostatic charges that develop on the external surface of the spacecraft, as well as provide passive thermal control. The charges would otherwise accumulate to cause arcing and possible damage to, or interference with, sensitive electronic equipment on or in the spacecraft. In order to dissipate electrostatic charge, the paint or film article must be somewhat electrically conductive, with a surface resistivity on the order of about 10.sup.9 ohms per square or less.
A spacecraft paint, whatever its color, should exhibit additional characteristics for spacecraft applications. The paint should be stable during long-term service in a space environment. The paint is desirably moderately tough and flexible so that it does not crack and flake away as it is flexed due to mechanical or thermal strains.
There are white, electrostatic-dissipative, organic-binder paints known for spacecraft use. The best of the known organic-binder paints, based upon the aluminum-doped zinc oxide pigment of the type disclosed in U.S. Pat. No. 5,094,693, whose disclosure is incorporated by reference, typically has a solar absorptance of from about 0.18 to about 0.22. No white, electrostatic-dissipative paints of lower solar absorptance are available. The lower the value of the solar absorptance, the lower the heating of the paint and thence the underlying substrate, in intense direct sunlight.
There is a need for improved white thermal-control paint and film articles that are operable and stable in a space environment, which have a low solar absorptance, and which can manage electrostatic discharge (ESD). The present invention fulfills this need, and further provides related advantages.